[Asterisk-biz] Modem Over IP: solutions ?

[Jean-Michel Hiver]

>not normally because of delay issues that arise with large jitter
>buffers.  Based on what the remote system says you might have to
>respond.  As such with a large jitter buffer you may be out of that
>window so your response is late and it messes stuff up.
>  
>
Hey I remember the pre-internet days when I used to download stuff off 
American BBS servers. It was done using a 14.4k modem and I'm sure the 
latency between France and the US was pretty high back then...


>Further if you use a codec that compresses highly you will corrupt the
>modem data becuase the output waveform is an approximation of the input
>not exactly the same.
>
Yes. I understand it's ulaw / alaw or nought.


>As such you cant use high compression algorithms reliably very well.
>  
>
Bandwith is not the issue there.


>FSK modems for example (used on the older and somewhat slower specs)
>transmit space and marks.  But to transmit more than 1 bit per iteration
>they will have ranges over and below the carrier freq.  So lets say for
>example +10Hz is bit pattern 10 +20Hz is bit pattern 11, -10 is 01 and
>-20 is 00.  There would be four frequencies that need to be represented
>accurately enough, generally not as big of a problem with slower modems,
>but it can be a problem once you get out of the FSK protocols, and with
>some of the higher speed FSK protocols.
>  
>
OK.


>Modems are very reliant (whether fax or not) on exact timings, jitter
>breaks that, and high lag induced by large jitter buffers can break
>response, so in short no a large jitter buffer doesnt always work.
>  
>
OK. So I guess I would have to study the effects of latency on the 
application.

I can guarantee that zere will be zero packet drop using VoIPoVPN and a 
large enough jitter buffer, so if the modem protocol supports 
satellite-like latency (that is about 500 ms) am I right understanding 
everything should be OK?


>You may get it to work some of the time, but with an alarm system would
>you want the liability of munged data not getting through and not being
>able to respond?  I wouldnt.
>  
>
Sure. But then it's not like the TDM network always works either - 
although it does most of the time :)


Cheers,
Jean-Michel.